In a disc drive, data is stored on one or more discs coated with a magnetizable medium. Data is written to the discs by an array of transducers, typically referred to as read/write heads, mounted to a radial actuator for movement of the heads relative to the discs. The data is stored on a plurality of concentric circular tracks on the discs until such time that the data is read from the discs by the read/write heads. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The heads are used to transfer data between a desired track and an external environment, which includes, among many components, a host computer. During a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. During a write operation, data is written onto the disc track. Once data is written to the disc, each sector holds a block of data, which is the absolute smallest quantity that can be written to the disc during a single write operation. Over the lifetime of a disc drive, data may be stored on a disc media for periods of time ranging from microseconds to any number of years.
With respect to data stored on disc media for relatively large periods of time, thermal decay of the disc media may eventually result in data loss of an undesirable magnitude. Thermal decay is a progressive loss of amplitude of recorded data on disc drive media. Thermal decay causes a loss of playback signal-to-noise ratio (SNR), which in turn causes degraded bit error rate (BER) of the recovered data. BER is defined as a ratio of error bits to total bits. As track widths have decreased, media grain sizes have been made smaller to boost SNR. Such a progressive decrease in media grain size typically results in higher levels of thermal decay on the disc media. Over an expected lifetime of five years, thermal decay typically causes a BER degradation of two orders of magnitude. That is, if a drive starts with a BER of one error in 108 bits read, which is conventionally denoted as BER=10−8, in five years the BER would most likely be one error for every 106 bits read, e.g., BER=10−6. Error rates below 10−6 have been shown to cause unrecoverable errors and drastically slow drive performance. Because future disc drives are expected to have even smaller grain sizes, present trends associated with increasing thermal decay are expected to continue.
Additionally, current difficulties in controlling distribution of grain sizes on disc media result in inconsistent levels of thermal decay on different disc surfaces, even within the same disc drive. Thermal decay of each particular disc may also be undesirably increased by SNR associated with a recording head playback element and a write field gradient of the write element, both of which can vary from head to head. As such, individual heads in a disc drive may also affect thermal decay variances between discs. Techniques do not currently exist at the media component level to economically and rapidly test each disc surface for BER degradation due to thermal and amplitude decay because a relatively significant amount of time is needed to reliably observe thermal decay in conventional disc drive products. Furthermore, thermal decay is typically greater at the high temperatures found in disc drives, such as temperatures realized during the operational state of the drive, and not the lower temperatures encountered during product development or functional testing processes.